Add hosted MCP server support with OAuth 2.1 and token refresh

Enables connecting to official MCP servers (like Notion) instead of
building custom WASM tools. Uses OAuth 2.1 with PKCE and supports
Dynamic Client Registration for zero-config authentication.

Key features:
- OAuth 2.1 flow with PKCE for secure browser-based auth
- Dynamic Client Registration (DCR) for servers without pre-configured clients
- Automatic token refresh on 401 responses
- Session management with Mcp-Session-Id headers
- SSE streaming response handling

New CLI commands:
- `mcp add <name> <url>` - Add an MCP server
- `mcp remove <name>` - Remove an MCP server
- `mcp list` - List configured servers
- `mcp auth <name>` - Authenticate with a server
- `mcp test <name>` - Test connection

Also removes the Notion WASM tool example since it's superseded by the
Notion MCP server which provides 13 official tools.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

CLAUDE.md

@@ -319,12 +319,106 @@ Key test patterns:
 
 ## Adding a New Tool
 
+### Built-in Tools (Rust)
+
 1. Create `src/tools/builtin/my_tool.rs`
 2. Implement the `Tool` trait
 3. Add `mod my_tool;` and `pub use` in `src/tools/builtin/mod.rs`
 4. Register in `ToolRegistry::register_builtin_tools()` in `registry.rs`
 5. Add tests
 
+### WASM Tools (Recommended)
+
+WASM tools are the preferred way to add new capabilities. They run in a sandboxed environment with explicit capabilities.
+
+1. Create a new crate in `examples/wasm-tools/<name>/`
+2. Implement the WIT interface (`wit/tool.wit`)
+3. Create `<name>.capabilities.json` declaring required permissions
+4. Build with `cargo build --target wasm32-wasip2 --release`
+5. Install with `ironclaw tool install path/to/tool.wasm`
+
+See `examples/wasm-tools/` for examples.
+
+## Tool Architecture Principles
+
+**CRITICAL: Keep tool-specific logic out of the main agent codebase.**
+
+The main agent provides generic infrastructure; tools are self-contained units that declare their requirements through capabilities files.
+
+### What Goes in Tools (capabilities.json)
+
+- API endpoints the tool needs (HTTP allowlist)
+- Credentials required (secret names, injection locations)
+- Rate limits and timeouts
+- Auth setup instructions (see below)
+- Workspace paths the tool can read
+
+### What Does NOT Go in Main Agent
+
+- Service-specific auth flows (OAuth for Notion, Slack, etc.)
+- Service-specific CLI commands (`auth notion`, `auth slack`)
+- Service-specific configuration handling
+- Hardcoded API URLs or token formats
+
+### Tool Authentication
+
+Tools declare their auth requirements in `<tool>.capabilities.json` under the `auth` section. Two methods are supported:
+
+#### OAuth (Browser-based login)
+
+For services that support OAuth, users just click through browser login:
+
+```json
+{
+  "auth": {
+    "secret_name": "notion_api_token",
+    "display_name": "Notion",
+    "oauth": {
+      "authorization_url": "https://api.notion.com/v1/oauth/authorize",
+      "token_url": "https://api.notion.com/v1/oauth/token",
+      "client_id_env": "NOTION_OAUTH_CLIENT_ID",
+      "client_secret_env": "NOTION_OAUTH_CLIENT_SECRET",
+      "scopes": [],
+      "use_pkce": false,
+      "extra_params": { "owner": "user" }
+    },
+    "env_var": "NOTION_TOKEN"
+  }
+}
+```
+
+To enable OAuth for a tool:
+1. Register a public OAuth app with the service (e.g., notion.so/my-integrations)
+2. Configure redirect URIs: `http://localhost:9876/callback` through `http://localhost:9886/callback`
+3. Set environment variables for client_id and client_secret
+
+#### Manual Token Entry (Fallback)
+
+For services without OAuth or when OAuth isn't configured:
+
+```json
+{
+  "auth": {
+    "secret_name": "openai_api_key",
+    "display_name": "OpenAI",
+    "instructions": "Get your API key from platform.openai.com/api-keys",
+    "setup_url": "https://platform.openai.com/api-keys",
+    "token_hint": "Starts with 'sk-'",
+    "env_var": "OPENAI_API_KEY"
+  }
+}
+```
+
+#### Auth Flow Priority
+
+When running `ironclaw tool auth <tool>`:
+
+1. Check `env_var` - if set in environment, use it directly
+2. Check `oauth` - if configured, open browser for OAuth flow
+3. Fall back to `instructions` + manual token entry
+
+The agent reads auth config from the tool's capabilities file and provides the appropriate flow. No service-specific code in the main agent.
+
 ## Adding a New Channel
 
 1. Create `src/channels/my_channel.rs`